1. Field of the Invention
The present invention relates to a steel cord for reinforcing rubber articles, and more particularly, to a steel cord-for reinforcing rubber articles capable of improving the resistance to corrosion propagation.
2. Description of the Related Art
Products reinforced with steel cords are liable to suffer from corrosion of steel filaments caused by water entering the products and thereby the durability and life of the products are lowered.
For example, when steel cords used in a belt of a tire with a void and the tire tread is subjected to damage reaching the belt, water entering the belt spreads along the longitudinal direction of the cord through the voids in the steel cords. As a result, rust formed due to water also diffuses and the adhesion between rubber and steel cord is lowered at that portion. Finally, separation phenomena occur.
In order to prevent such corrosion propagation, there is proposed a cord structure in which rubber can sufficiently penetrate into the inside of the cord through gaps between adjacent metal filaments by curing process.
Japanese Patent Publication Nos. 21641/1987 and 49421/1985 disclose that gaps between filaments of a cord can be formed by excess shaping. However, retaining such gaps is difficult and moreover the advantageous effect varies undesirably depending on the manner of handling in the step of manufacturing tires.
On the contrary, there are proposed techniques for assuring gaps between filaments by improving the cord structure without shaping filaments.
Japanese Patent Application Laid-open Nos. 38208/ 1985 and 1790/1984 disclose that one of the above-mentioned cord structure, so-called "(1+5) structure" cord composed of one core filament and five sheath filaments, has gaps between sheath filaments and rubber can easily penetrate the gaps, and further this cord can be produced by one-step twisting. As a result productivity is high.
Indeed the average sheath gaps are sufficient in such a cord structure, but deviation occurs in the arrangement of sheath filaments and there are formed attaching portions of the filaments resulting in forming the portions where rubber does not penetrate due to fluctuation in the manufacturing procedure.
Japanese Patent Application Laid-open No. 175503/ 1989 proposed a steel cord composed of one core filament and six sheath filaments, and Japanese Utility Model Application No. 178204/1985 and Japanese Patent Application Laid-open No. 154086/1990 disclose two-layer twisted steel cords composed of a core of two metal filaments and a sheath filament disposed around the core.
In particular, the (1+6) structure cord of the above-mentioned Japanese Patent Application Laid-open No. 175503/ 1989 can be produced by one step twisting and therefore, is advantageous from the standpoint of productivity. The disclosed (1+6) cord has a structure such that the diameter of the core filament is larger than that of the sheath filament so as to ensure a gap larger than a predetermined size between adjacent sheath filaments for enabling rubber to penetrate the inside. However, since partly a deviation occurs in the arrangement of sheath cores, rubber does penetrates at the side where sheath filaments contact each other and therefore, a sufficient resistance to corrosion propagation can not be attained in the case of tires for trucks running a severely rough road with much water. Further, the weight of cord is large and the productivity is lowered.
Japanese Patent Application Laid-open No. 131404/ 1981 discloses that a cord of (1+5) structure may be formed such that the core filament is lightly waved, but in this cord structure the diameter of the core filament is thinner than that of the sheath filament and therefore, the distance between sheath filaments is so narrow that rubber can not easily enter the gap, and further, since rigidity of core filament is so low that the wave shape of the core exhibits only a lowered effect. In addition, when the core shaping ratio (waving) is large, the strength is lowered.
It may be thought that a diameter of the core filament is made larger than that the sheath filament to assure a gap larger than a predetermined size between adjacent filaments for purposes of making rubber penetrate the inside. However, such method increase the total weight of the cord, the productivity is deteriorated and deviation of the arrangement of sheath filament occurs partly to cause attachment between sheath filaments each other. As a result, rubber can not penetrate the cord resulting in less resistance to corrosion propagation,